<!DOCTYPE html>
<html>
<head>
<script src="http://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(function(){  
    var t = $("#text_box");  
    $("#add").click(function(){       
        t.val(parseInt(t.val())+1)  
        setTotal();  
    })  
    $("#min").click(function(){  
        t.val(parseInt(t.val())-1)  
        setTotal();  
    })  
  function setTotal(){  
        $("#total").html((parseInt(t.val())*300).toFixed(2));  
    }  
    setTotal();  
})  
</script>  
</head>  
<body>  
  
<input id="min" name="" type="button" value="-" />  
<input id="text_box" name="" type="text" value="4" />  
<input id="add" name="" type="button" value="+" />    
  
</body>  
</html>